Sedimentology is the study of sedimentary rocks that are formed by: (1) the deposition of rock fragments which have been transported from their source to another location by water or air (sandstone and shale); (2) precipitation from a liquid or solution (salt, gypsum); and (3) remains (shells, skeletons and organic matter) of organisms (limestone, coal). Sedimentary rocks are deposited in layers known as strata. Stratigraphy is the study of the origin, composition, distribution and succession of these strata.
Oil and gas reservoirs occur in sedimentary basins. To recover the petroleum from these reservoirs typically requires drilling through thousands of feet of overlying rock. The drilling of oil and gas wells is normally a very expensive endeavor. Consequently, before incurring this expense, those involved in the exploration for or exploitation of oil and gas reservoirs seek to obtain an understanding of the basin geology and, in particular, the basin sedimentology and stratigraphy so that an oil/gas well is drilled in the location that is likely to achieve the desired result. In the case of oil and gas exploration, geologic and seismic data are used to predict the location of sedimentary rocks and structures that are likely to contain an oil/gas reservoir. With respect to developing an oil/gas reservoir, geologic and seismic data are used to predict locations for drilling wells that will facilitate the extraction of more oil from the reservoir.
Presently, there are many different techniques available for obtaining sedimentologic and stratigraphic data. One technique is seismic surveying, which involves: (1) transmitting sound waves from the surface into the earth; (2) recording the waves that are reflected back to the surface when the transmitted wave encounters interfaces between strata, fractures and the like in the underlying earth; and (3) analyzing the reflected signals to make geological inferences about sedimentary rocks and fluids encountered by the waves as they propagate through the earth. Other techniques that are also used are coring and well logging, which involves taking samples of the various rocks and fluids encountered as a well is drilled, noting the extent of each particular kind of rock that is encountered during the drilling, and inserting various instruments into the well that measure various rock and fluid properties, such as porosity.
Most of the techniques for obtaining sedimentologic and sedimentary data are also relatively expensive and are limited to the locations in which the data are taken, i.e. the data obtained by one of these techniques at a particular location are not representative of the underlying geology for more than a short distance away from the location at which the data were taken. As a consequence, any conclusions drawn with respect to sedimentological and sedimentary attributes are subject to increasing uncertainty as the location of interest becomes increasingly remote from the locations at which the data are taken.